In a typical turbine generator system, a series of turbines are utilized to produce relative rotational movement between an armature and field for producing electricity. It is known that during generation of electricity, various charges build up on the turbine-generator shaft. The charges can cause arcing from the shaft to the bearings of the turbine-generator, which after a prolonged period can result in the bearings inability to maintain a sufficient film of oil for support of the shaft. When the bearing is unable to maintain an adequate oil film, catastrophic failure occurs in the form of a bearing wipe.
In a turbine, a shaft is provided for transmitting mechanical power, with the shaft passing from a high pressure internal turbine environment to the relatively low pressure ambient. To isolate the high pressure interior, a seal is provided about the shaft. In older turbines, a water seal was utilized, with water pumped under pressure about the shaft, thereby forming a fluid seal about the shaft to isolate the turbine interior from the ambient. While it was generally not recognized, the water also served to conduct current and charges away from the shaft, thereby preventing charges from accumulating on the shaft. In more recent turbines, steam, rather than water, has been utilized for providing a fluid seal. While steam is effective in providing a pressure seal, it is ineffective in dissipating charges which accumulate on the turbinegenerator shaft. In many installations, the problems associated with accumulation of charge on the generator shaft were not recognized, and bearing wipes, or burning out of the bearings occurred.
In the turbine bearings, a thin film of oil, on the order of 5-6 mils, supports the shaft or journal for rotation in the bearing. Generally, very small flakes of metal are also present in the lubrication oil. The metal flakes can aid in arcing across the film when a charge accumulates on the shaft. When arcing occurs, the bearing surface is degraded (often referred to as frosting), reducing the ability to maintain an adequate lubrication film thickness. The degradation of the bearing generally goes unnoticed, until a catastrophic bearing wipe occurs, resulting in complete destruction of the bearing and requiring downtime of the system for costly repairs.
Charges can accumulate on the generator shaft from a number of sources. The largest sources are high impedance electrostatic charges, and low impedance dissymmetry voltages. Smaller charges are also present, such as a high impedance exciter voltages, which are produced as large spikes resulting from exciter current switches, and small homopolar currents, which are generated in the bearing. The latter may be easily eliminated by demagnetizing the generator shaft. The high impedance charges result from an electrostatic charge transfer to the shaft. The low impedance, higher current sources, typically dissymmetry voltages are built up within shaft within the generator and must be prevented from reaching the turbines/bearing sites.
In order to prevent the accumulation of charges, brushes have been provided on the generator shaft between the final (low pressure) turbine stage and the generator. The brushes remove the electrostatic charges, and establish a zero point for the dissymmetry voltage. In establishing the zero point of the dissymmetry voltages, arcing at the turbine bearings is prevented, since the charges will be zero along the shaft from the brush to the turbine governer end. While the dissymmetry voltage will still be present extending from the brush to the generator and the exciter, the generator and exciter bearing pedestals are insulated. The provision of insulated bearings for the turbines is too costly, particularly since more bearings are needed for the turbines than for the generator and exciter.
The charges on the shaft build up quite rapidly. For example, the electrostatic charges will have about 10-20 milliamps current and can accumulate charge at a rate of 30,000 volts per second. The dissymmetry voltages will typically be 10-20 volts, but may be as high as 100 volts, and can have a high associated current. A shaft voltage of 10 volts or even less can be sufficient to cause arcing. Thus, when a grounding brush leaves the shaft for only a short time arcing can occur. It is therefore extremely important to provide substantially continuous contact of the brush with the generator shaft.
In existing brush arrangements, often a brush holder arm is provided which is mounted upon a stud. The brush arm includes a box portion through which the brush is slidably mounted, with a spring resiliently urging the brush towards the shaft. Due to vibrations imparted to the stud, and slight wobbling of the shaft, the brush often bounces away from the shaft, with the spring acting to return the brush to the shaft. A typical turbine generator system will include a pair of brushes, so that one of the brushes can ground the shaft even if the other is temporarily bounced from the shaft. However, the brushes can be bounced from the shaft at the same time, particularly since the studs upon which the brush holder arms are mounted can often vibrate in phase.
In the conventional system, while the spring force causes the brush to regain contact with the shaft, friction associated with the sliding mount of the brush can slow the ability of the brush to return to the shaft and reestablish contact. The slowness of return can be sufficient to allow charge to accumulate and cause arcing. Due to the rapid build up of charge on the shaft, it is preferable to maintain the period of time during which the brush is out of contact to a maximum of 100 microseconds. In the conventional brush arrangement, often the brush is out of contact for periods on the order of milliseconds or large fractions of milliseconds. The occurrence of arcing is generally unnoticeable, as is the slow degradation in the bearing surface. However, after a prolonged period of arcing, the bearing will be unable to provide a sufficient oil film and a catastrophic failure can result.
Thus, a brush which is capable of removing charges from the generator shaft is needed, in which the period of time in which the brush is out of contact with the generator as a result of brush bounce is minimized, thereby eliminating or reducing arcing in the bearings.